Among oxide superconductor tapes, the YBa2Cu3O7−x (hereinafter referred to as YBCO for convenience) superconductor tape generally has a structure in which one or, more biaxially oriented inorganic thin film layers are formed on a metallic substrate, and a superconductive film and stabilization layer are formed sequentially thereon. Since its crystals are biaxially oriented, this tape has a higher critical current value (Ic) than bismuth-class Ag sheathed tape, and has excellent magnetic field properties at the liquid nitrogen temperature, so that use of this tape offers the advantage of enabling superconductor devices currently used at low temperatures around the liquid helium temperature to be used at elevated temperatures.
Also, since the superconductive properties of an oxide superconductor change according to its crystal orientation, in order to improve Jc it is necessary to improve the in-plane crystal orientation and thus to form an oxide superconductor on a tape-shaped substrate. Consequently, a film deposition process is used in which an oxide superconductor is epitaxially grown on a substrate having high in-plane crystal orientation.
In this case, in order to improve Jc, it is necessary to maintain good superconductive state quantum bonding by orienting the c-axis of the oxide superconductor vertically with respect to the surface of the substrate and orienting the a-axis (or b-axis) of the oxide superconductor in plane so as to be in parallel to the substrate surface. Consequently the in-plane orientation and c-axis orientation of crystals in the superconductive layer crystal have been improved by forming a buffer layer that has improved in-plane crystal orientation and c-axis crystal orientation on a metallic substrate having high in-plane crystal orientation, and using the crystal lattice of this buffer layer as a template. Also, in order to improve the Ic value, it is necessary for the oxide superconductor film formed on the substrate to be made thicker.
The conduction characteristic (Jc) of a superconductor depends on the crystallinity and surface smoothness of an buffer layer, and it has been proved that this characteristic changes sensitively according to the state of the base.
Various film deposition methods are currently being studied for YBCO superconductor tapes, and IBAD (Ion Beam Assisted Deposition) and RABiTs (registered trademark: Rolling Assisted Biaxially Textured Substrate) are known as manufacturing technologies for a biaxially oriented metallic substrate whereby an buffer layer with in-plane oriented crystals is formed on a tape-shaped metallic substrate used therefor. Many YBCO superconductor tapes have been reported in which an buffer layer that has improved in-plane crystal orientation and c-axis crystal orientation is formed on a non-oriented or oriented metallic tape. For example, a rare-earth-class tape-shaped superconductor is known in which a substrate of Ni or an Ni-based alloy having an orientation texture through heat treatment after rolling is used as a substrate, and an Ni-oxide thin layer, an oxide buffer layer such as a CeO2 (cerium oxide) or like layer formed by means of MOD (Metal Organic Deposition) process, and a YBCO superconductive layer, are formed sequentially on the surface thereof (see Patent Literature 1, for example).
Of these, the method using an IBAD substrate has enabled the highest performance to be obtained. In this method, a buffer layer (CeO2, Y2O3, YSZ, or the like) or two-layered buffer layer (YSZ, Gd2Zr2O7/CeO2, Y2O3, or the like), which has high crystal orientation and suppresses reactions with constituent elements of a superconductor, is deposited on a high-strength, nonmagnetic tape-shaped Ni-class substrate (such as hastelloy) by laser deposition in which particles generated from a sputtering target is deposited while irradiating the substrate with ions from a direction diagonal to the substrate, followed by deposition of CeO2 thereon by means of PLD (Pulsed Laser Deposition) and deposition of a YBCO superconductive layer on CeO2 by means of PLD (see Patent Literature 2, for example), to form a superconductor tape. Below, Gd2Zr2O7 is referred to simply as GZO.
CeO2 is used as a buffer layer of a Y-class superconductor. A CeO2 buffer layer is known as one of the best buffer layers because it has good bonding with a YBCO superconductive layer, and is less reactive with a YBCO superconductive layer.
The roles of a CeO2 buffer layer constituting the base of a superconductive layer include providing good lattice compatibility between an oxide superconductor layer and GZO buffer layer, suppressing elemental diffusion of the metallic substrate, and so forth. It is known that the crystal grain orientation of the CeO2 buffer layer greatly affects the crystal orientation and critical current value (Ic) of the superconductive layer above. That is to say, the superconductive properties of the YBCO film are greatly influenced by the in-plane crystal orientation and surface smoothness of the CeO2 buffer layer.
In the CeO2 buffer layer manufacturing process, PLD, with which there is little compositional deviation between a target and a manufactured film, and that enables film formation in a high-oxygen concentration atmosphere, is used for oxide film formation. Also, secondary effects of using PLD include self-epitaxy, i.e., the in-plane crystal grain orientation (Δφ) of the CeO2 film sharply increases with increasing thickness, and high-speed film deposition, when the CeO2 film is formed on the GZO buffer layer.